command/fmt: Disable list/write when using STDIN

These options don't make sense when passing STDIN. `-write` will raise an
error because there is no file to write to. `-list` will always say
`<standard input>`. So disable whenever using STDIN, making the command
much simpler:

    cat main.tf | terraform fmt -

command/fmt.go

@@ -51,7 +51,10 @@ func (c *FmtCommand) Run(args []string) int {
 	var dirs []string
 	if len(args) == 0 {
 		dirs = []string{"."}
-	} else if args[0] != stdinArg {
+	} else if args[0] == stdinArg {
+		c.opts.List = false
+		c.opts.Write = false
+	} else {
 		dirs = []string{args[0]}
 	}
 
@@ -76,9 +79,9 @@ Usage: terraform fmt [options] [DIR]
 
 Options:
 
-  -list            List files whose formatting differs
+  -list            List files whose formatting differs (disabled if using STDIN)
 
-  -write           Write result to source file instead of STDOUT
+  -write           Write result to source file instead of STDOUT (disabled if using STDIN)
 
   -diff            Display diffs instead of rewriting files
 

command/fmt_test.go

@@ -137,11 +137,7 @@ func TestFmt_stdinArg(t *testing.T) {
 		input: input,
 	}
 
-	args := []string{
+	args := []string{"-"}
-		"-write=false",
-		"-list=false",
-		"-",
-	}
 	if code := c.Run(args); code != 0 {
 		t.Fatalf("wrong exit code. errors: \n%s", ui.ErrorWriter.String())
 	}

website/source/docs/commands/fmt.html.markdown

@@ -22,6 +22,7 @@ input (STDIN).
 
 The command-line flags are all optional. The list of available flags are:
 
-* `-list=true` - List files whose formatting differs
+* `-list=true` - List files whose formatting differs (disabled if using STDIN)
-* `-write=true` - Write result to source file instead of STDOUT
+* `-write=true` - Write result to source file instead of STDOUT (disabled if
+    using STDIN)
 * `-diff=false` - Display diffs instead of rewriting files